What happened
On 24 August 2006, a Reims Cessna F152, registration G-BMCV, was conducting a solo VFR navigation training exercise from Leicester Airport. The flight plan involved flying via Sywell and Conington before returning to Leicester.
While climbing through approximately 2,000 ft within the Leicester airfield overhead, the student pilot observed the engine noise decreasing and the indicated RPM dropping. Despite the throttle remaining in the fully open position, the engine lost power and the aircraft ceased its climb. The pilot declared a ‘MAYDAY’ and transitioned to a final approach. Although the pilot successfully performed a forced landing on the runway, the aircraft overshot the end of the runway and came to rest in an adjacent field. There were no injuries to the pilot, and the aircraft sustained damage to the propeller and the right wing.
The investigation
Investigators examined the engine and performed a metallurgical analysis of the damaged components. The engine, which had flown 1,293 hours since its last overhaul, was dismantled by an overhaul agency.
Examination of the No 4 exhaust cam follower revealed that the head had broken away from the shaft, with fragments found in the oil sump. Microsection analysis of the cam follower material showed that the component had been properly carburised and case-hardened. However, the investigation identified that the camshaft surface at the No 4 lobe had suffered mechanical damage. Additionally, the camshaft bearing faces and lands were found to be scored, which was attributed to the circulation of metal debris within the oil. The crankcase also showed mechanical damage on the right side, consistent with high-energy contact with the broken cam follower pieces.
Findings
- The loss of power was caused by the break-up of the No 4 cylinder cam follower.
- The failure of the cam follower was driven by a flexural fatigue mechanism.
- This fatigue was the result of offset cyclic loading caused by the associated camshaft lobe.
- Debris from the failed component circulated through the engine, causing secondary damage to the camshaft bearings and the crankcase.